import re
txt='Line 1 with a hard line break\nLine 2 with a hard line break\nLine 3 with a hard line break\n\nLine 4with a hard line break with a double newline for the paragraph break\nLine 5 with a hard line break\n'
print txt
print 
print 'replaced:'
print re.sub(r'(?<!\n)\n(?!\n)(?m)', '', txt)

Line 1 with a hard line break
Line 2 with a hard line break
Line 3 with a hard line break

Line 4with a hard line break with a double newline for the paragraph break
Line 5 with a hard line break


replaced:
Line 1 with a hard line breakLine 2 with a hard line breakLine 3 with a hard line break

Line 4with a hard line break with a double newline for the paragraph breakLine 5 with a hard line break